1. Field of the Invention
The present invention relates to an improved method for removing traces of heavy metals from concentrated alkali metal chloride solutions, particularly to bring the level of such heavy metals down to below the level of detectability into the range of 0.01 ppm or less.
2. Discussion of the Background
During the operation of a chlorine-alkali-electrolysis according to the amalgam process, the presence of heavy-metal ions may give rise to serious problems. These metals cause a reduction in the hydrogen overpotential, so that hydrogen is generated instead of amalgam being formed. Even small amounts of iron, cobalt, nickel or tungsten cause an increased hydrogen content in the chlorine. Small amounts of calcium, magnesium, aluminum or barium can also cause an increase in the hydrogen content in the chlorine, as well as higher amounts of sodium and potassium (&gt;0.5%).
The most serious effects, however, are observed with vanadium, molybdenum and chromium. These metals have a strong effect even in trace amounts (ppb). During the process, the elements Cr, Ni, V and Mo tend to accumulate in the working brine, since they or their compounds are unaffected by the conventional alkaline carbonate precipitation and since the brine is recirculated.
The principle of picking up traces of less readily precipitable substances by means of coprecipitation is known from radiochemistry and nuclear chemistry and from other fields (Kirk Othmer (3rd), 19, 692-228). In water/ wastewater chemistry the coprecipitation of traces of heavy metals is covered by the prior art (Ullmann (4th) 6, 441 et seq.). In particular it is reported, for example in Environmental Science & Technology (9th) 8, 744 et seq.! that it was possible to reduce the molybdenum level of river water by 95% to 0.05 ppm by means of precipitation at pH values between 3 and 4.
However, such a process has never before been successful in treating solutions with high levels of alkali metal chlorides, such as are produced by the conventional chlorine-alkali electrolysis process.